Russian opposition leader Alexei Navalny completed a 15-day jail sentence on Monday. By Tuesday, he was promising to organize more of the massive protests that got him arrested.

“I, of course, assess the March 26 action to be very successful,” Navalny told supporters in an online message transcribed by Reuters. “It was the first simultaneous action in towns since the 1990s. Despite the fact that the authorities tried to frighten everyone… tens of thousands took to the streets. We need to continue.”

He was also proud of the popularity of his documentary alleging corruption by Russian Prime Minister Dmitry Medvedev. The online film was a driving force behind the March demonstrations and has reportedly accumulated 18 million views online.

It might also cause a bit of legal trouble for Navalny, as another subject featured in the film, billionaire Alisher Usmanov, announced on Tuesday that he will sue Navalny for slander. The film alleges Usmanov gave donations of land and houses to a foundation as a means of bribing state officials to obtain special favors.

Navalny evidently still intends to run against Vladimir Putin for president next year, despite polls that give him little chance of success and a court order effectively barring him from running because he was found guilty of defrauding a state company.

His campaign chief said the protests “showed that Alexei Navalny is the politician who feels the political sentiment in the country most keenly.” According to the Associated Press, Navalny’s campaign already has offices in over two dozen cities all across Russia and plans to have over forty offices open by the end of May. He is hoping to tap into marginalized voters disgruntled with the Putin government but worried that not enough people agreed with them to make Putin’s defeat at the ballot box possible.

The European Union pressured Russia to release Navalny and the other thousand or so activists arrested during the protests with him. The U.S. State Department denounced these arrests as an “affront to core democratic values.”